Rapid-wetting gypsum-base backfill for cathodic protection



Aug. 23, 1949. M. o. ROBINSON ET AL RAPID-WETTING GYPSUM- BASE BACKFILL FOR CATHODlC PROTECTION Filed Jan. 7, 1948 Magnesium an oo/e 1N V EN TORS. Me/v/'n O. 'IQOb/'nSOn BY Ham/dA. Ro/'nson A TTORNEYS Patented Aug. 23, 1949 I TED PATENT OFFICE ompany, 'Midland Bela'vvafe Mich., a corporation of Applica-tien January 7, 194s, serial No. 982

This .invention relates to improvedanode media for use in the galvanic protection of underground metals. It also concerns packaged anodes containing such media and protection systems using them. A I

`In galvanic systems for the cathodic protection of pipelines and other underground structures, sacrici-al `anodes of a vmetal electronegativeto thestructure., usually magnesium or zinc, are buried in the earth near the structurel and connected to it by electrical conductors. The resulting iiow of current maintains the structure cathodic with respect to the .soil and greatly minimizes its corrosion. In such service, itis often desirable, when using magnesium metal as a sacricial anode, t bury itin a prepared bed or baclriill designed to control the chemical nature oi the anode environment. .l

The chemical requirements f an idealr backll for magnesium anodes are various. In use,

the material should have goed electrolytic con'- ductivity and yet should not be so soluble' as to' be leache'd away. It should be capable of wetting easily in g'rund' waters, and should retain moisture during dry spellsv lso as to avoid loss of condu-ctiv'ity'f.Y It should also maintain a nearly constant volume, i. e.A should not swell and then shrink away from the an'd'e, during seasonal increase and decrease the moisture content of the surrounding earth. Further', in order to secure maximum `current efficiency from the magnesium, the backll should tend to minimize l'- calized, i.- e.; useless; corrosion of the metal.l On the other hand, it should promote uniformity of attack during the useful or current-producing consumption oi the magnesium. In addition, .it must not polarze the anode undulyV or form impervious coatings on it.

These specifications are met only in part by mixtures of bentonite with a lesser proportion of gypsum, which have been the principal backi-llls used heretofore. In particular, these mixtures are disadvantageous in that they absorb ground water very slowly in service, and hence do not permit development of full galvanic current for several days after installation. In addition, they shrink unduly during dry weather, markedly increasing the electrical resistance of the anode circuit at such times.

It is therefore the principal object of the present invention to provide an improved backll for use as an anode medium in cathodic protection systems which is rapid-wetting and which undergoes comparatively little variation in volume with changes in moisture content. Another object is Vto `provide a backll composition in.r the form of a free-owing powder particularly useful in rnaking packaged anodes. y

The invention will be explained with reference to the accompanying drawing, in which v Fig. i is a schematic vertical section showing one manner of using the new composition as an anode backll in the galvanic protection of a buried pipeline; and Y i Fig. 2 is a Verticai section through a packaged anode. if

The compositions of the invention consist essentially of intimate mixtures of from 70 to 89 percent by weight of powdered gypsum, from 10 to 25 percent of powdered bentonite, and from 1- to 5 percent of a water-soluble salt of a metal at least as anodic as magnesium, preferably sodium sulfate. Y

The gypsum used, i. e. Ca'SO42H2O, is coneni-ently the naturally-occurring substance ground to a powder state. While dusting gypsum is operable, it is preferable to use a somewhatY coarser grade, most satisfactorily one having a degree of fineness such that at least 60 a percent by weight corresponds to a screen analysis of 20 to 100 mesh (Tyler Standard). A preferred gypsum is one in which from to 10 percent is retained on a 20 mesh screen, from 60 to 7U percent on Se mesh, and not over 25 perceitner than Q mesh.

The term bentonite as used herein, refers tbth'e' naturally-occurring colloidal clay or volcanic' ash consisting largely of one of the minerals sodium montmorillonite and beidellite and 3;, being characterized by swelling extensively in water. This product is sometimes called alkali bentonite in contrad'i'stinction to so-lcalled alkaline earth bentonite which is almost nonswelling in water and is not operable in the present composition. In the invention, it is preferable to employ a bentonite ground somewhat less ne than the most common commercial grades. A particle size such that at least 60 percent by weight of the bentonite corresponds to a screen analysis between and 100 mesh is most satisfactory.

In addition to gypsum and bentonite, the backllls of the present invention also contain a small proportion of a water-soluble salt of a metal at least as anodic as magnesium. Since the function of this salt is simply to insure high electrolytic conductivity in the backll, a, variety of salts may be used, e. g. magnesium sulfate, sodium chloride, potassium sulfate, etc. However, sodium sulfate, usually as the anhydrous salt, is greatly preferred 3 because it tends to insure uniformity of attack on the magnesium anode buried in the backiill.

The relative proportions of the gypsum, bentonite, and sodium sulfate must be carefully controlled within the limits stated to secure a rapid-wetting, low-swelling composition. Optimum results appear to be obtained with about '75 percent by weight of gypsum, about 20 percent bentonite, and about percent sodium sulfate. This mixture has an apparent specific gravity of 1.29 and a specific resistivity when wet of 35 to 50 ohms per cubic centimeter. When the dry mixture is immersed in water, the apparent increase in Volume is only 32 percent, as contrasted to far higher values, often over 100 percent, for the backll compositions heretofore used.

If desired, small proportions of inert substances, such as sand, may be incorporated in the new backflll compositions. In general, however, such fillers offer no advantage and are omitted.

The new compositions are made by simply mixing the ingredients together thoroughly, as with a rotating tumbler, each ingredient having been previously ground to the appropriate neness. The resulting mixture is extremely free-flowing.

The manner of using the new compositions as a cathodic protection backiill is illustrated in Fig. 1, in which a steel pipeline 4 buried in earth is being protected. The consumable galvanic anode 5 is an elongated cylindrical body of magnesium metal provided with a central iron pipe core 6. As shown, the anode is buried in the earth near the pipeline, with the core 6 being connected electrically to the line by a conductor l. A bed of wetted gypsum-bentonite-sodium sulfate mixture 8 surrounds the anode and is in rm contact with it and with the earth.

In making the installation, a suitable hole is dug and the anode is lowered in place, after which the backll is tamped around it. The electrical conductor to the pipe-line is then installed and buried. In dry soils, water is poured around the anode and backiill to hasten the beginning of electrolytic action.

An optional method of installation, which is particularly convenient under many conditions, involves the packaged anode illustrated in Fig. 2. In this case, a magnesium anode 5 is centered in a water-permeable container, such as a paper carton 9. The space between the anode and the container walls is then lled with a mass 8 of the backi'lll composition according to the invention. In installing this anode, it is necessary only to dig a hole the size of the carton 9, insert the entire package, tamp the earth around it, and make the necessary electrical connection.

In eld use of the invention, the number and 4 size of anodes and the quantity of backll rquired to secure eiective cathodic protection of a given pipeline or other structure are determined by well-known engineering principles.

While the invention has been described as useful in the cathodic protection of underground ferrous metal structures,.it is applicable in protecting underground structures of any corrodible metal cathodic to magnesium. The sacrificial anodes may be made either of magnesium or a magnesium-base alloy, both being comprehended by the term magnesium metal as used herein.

What is claimed is:

1. A composition useful as an anode medium in cathodic protection and consisting essentially of a mixture of from to 89 percent by weight of powdered gypsum, from 10 to 25 percent of powdered bentonite, and from 1 to 5 percent of a water-soluble salt of a metal at least as anodic as magnesium.

2. A composition according to claim l wherein the salt is sodium sulfate.

3. A composition according to claim 2 wherein the gypsum and the bentonite each is of a degree of iineness such that at least 60 percent by weight thereof corresponds to a'screen analysis between 20 and 100mesh.

4. A composition consisting of a mixture of about percent by weight powdered gypsum, about 20 percent powdered bentonite, and about 5 percent sodium sulfate.

5. A packaged anode for use in cathodic protection systems comprising a water-permeable container having therein a magnesium metal anode provided with means for connecting an electrical'conductor thereto, such anode being surrounded by and in intimate contact with a mass of the composition deiined in claim l.

6. In combination with an underground structure of a metal cathodic to magnesium, a cathodic protection system comprising a magnesium metal anode buried in the earth near the structure and electrically connected thereto, such anode being surrounded by and in intimate Contact with a bed of backll material as dened in claim 1.

" MELVIN O. ROBINSON.

HAROLD A. ROBINSON.

REFERENCES CITED The following references are of record in the le of this patent:

1 The Petroleum Engineer, Aug. 1946, pages 136 through 140, article by Hart et al. 

